Potatoes (Solanum tuberosum) are an important crop plant, particularly in the Pacific Northwest (Washington, Oregon, and Idaho), where they are grown on over 500,000 acres. In addition, Idaho has a high value seed potato industry.
Weeds are the most important biotic stress factor limiting potato yield (Rioux et al. (1979) Can. J. Plant Sci. 59:367-374). Weeds compete with potatoes for light, water, and nutrients, and interfere with harvest operations. For example, one redroot pigweed (Amaranthus retroflexus) per meter of row reduces marketable potato yield by 28 percent, and one barnyard grass plant per meter of row reduces marketable yield by an average of 20 percent (Vangessel and Renner (1990) Weed Sci. 38:338-343). In addition, weeds harbor pests that attack potatoes, including insects, nematodes, and common potato diseases such as early blight, late blight, Verticillium wilt, and potato viruses X and Y (Ogg and Rogers (1989) Rev. Weed Sci. 4:25-58).
Mechanical removal by surface tillage and chemical removal by application of herbicides are the conventional weed control options in potatoes (Dallyn (1973) Am. Potato J. 48:116-128). Strict tuber quality requirements limit the use of mechanical cultivation for weed control. Multiple cultivations compact soil, which reduces aeration and potato growth and produce clods that bruise potatoes at harvest (Callihan (1989) Univ. of Idaho Bulletin No. 695). Cultivation may also directly damage foliage and roots, reducing yield and decreasing tuber quality. Growers using cultivation alone for weed control have been shown to have a 12 to 20 percent yield loss compared to growers using herbicides (Callihan (1989) Univ. of Idaho Bulletin No. 695). In seed growing areas, cultivation after potato emergence may spread disease.
Although potatoes are a major crop in the Pacific Northwest, they are viewed as less significant from a nationwide perspective by agricultural chemical companies. Consequently, few herbicides are registered for use on potatoes and very few experimental compounds are being evaluated. Only six herbicides, trifluralin, pendimenthalin, EPTC, metolachlor, metribuzin, and sethoxydim are currently registered for use on potatoes in Idaho and Washington. Of these, metribuzin is used on the majority of acres treated because the other available compounds are primarily (or solely in the case of sethoxydim) grass-control herbicides. In contrast, metribuzin controls a broad spectrum of broadleaf and grass weeds that are problems in potato production. Metribuzin may be used before or after emergence of the potato plants and is the only herbicide available for postemergence control of broadleaf weeds in potatoes.
Heavy reliance on metribuzin for weed control in potatoes has led to a shift to weed species that are metribuzin-tolerant, such as hairy nightshade (Solanum sarrachoides). Weed populations resistant to triazine herbicides have developed, such as the triazine resistant Powell amaranth population recently discovered in south central Idaho (Eberlein et al. (1992) Weed Sci. 40:507-512). Metribuzin suffers from additional drawbacks. It is a moderately residual herbicide that may persist to injure cereal crops grown in rotation with potatoes. Metribuzin is also readily leached in the low organic matter sandy soils typical of Washington's potato producing areas, and moderately leached in the low organic matter, silt loam soils typical of Idaho's major potato producing areas. In Herbicide Handbook of the Weed Science Society of America, 6th ed., pp. 183-185). Groundwater contamination with metribuzin has also been detected in the irrigated sands used for potato production in Wisconsin (Binning, 1990) and Minnesota (Wyse, D. L., Univ. of Minnesota, St. Paul, Minn., personal communication).
These environmental drawbacks further detract from the use of metribuzin to control weeds. Moreover, an exclusive reliance on chemical herbicides may be restricted in the near future by increasingly stringent regulatory controls. Furthermore, use of chemical herbicides for weed control prevents the potato crop from being sold in the increasingly important "organic produce" consumer market.
There is therefore a clear need for alternative practices for weed control in potato cultivation. Moreover, this need extends to almost all crops grown in the United States today. The search for new weed control technologies has in part focussed on biological control mechanisms. One such option is the use of competitive cover crops.
A cover crop is a crop that is planted either before or at the same time as an annual crop that is to be harvested (herein referred to as the "production crop"). A cover crop can also be used with perennial production crops; again, the cover crop is planted annually. A cover crop can reduce soil erosion that typically occurs in unplanted fields and can also reduce the growth of weeds in the field. Cover crops have been successfully used for weed control in several vegetables in Michigan (Putnam et al. (1983) J. Chem. Ecology 9:1001-1010) and in corn (Zea mays) and soybeans (Glycine max) in Minnesota (Warnes et al. (1989) Effect of precipitation and management on weed control and crop yields with a winter rye cover crop system, Agronomy Abstracts, p. 296). In Michigan, fall-planted wheat (Triticum aestivum) or rye (Secale cereal) killed with an herbicide the following spring reduced weed biomass 84 percent and 56 percent, respectively. In Minnesota, fall-planted rye killed chemically the following spring gave about 30 days control of small-seeded weeds such as kochia, redroot pigweed, common lambsquarters, and green foxtail.
To prevent competition between the cover crop and the production crop, the cover crop needs to be killed or removed from the field prior to the growth of the production crop. Two approaches to this have been used to date; mechanical removal of the cover crop with farm implements and chemical killing of the cover crop using herbicides. Both of these approaches have drawbacks: mechanical removal requires additional ploughing of the fields and herbicide application re-introduces chemicals that the cover crop approach was intended to avoid. Removal of the cover crop with herbicides may also mean that the production crop cannot be sold as an organic product. Because of these drawbacks, cover crops are used on only a limited scale at present.
U.S. Pat. No. 5,278,057 (Jorgensen, 1994) discusses cover crops into which a dominant conditional lethal marker has been introduced by genetic engineering techniques. Such a cover crop can be killed to free a field for replanting by application of a chemical compound which is converted by the marker gene into a toxic metabolite. Here again, the removal of the cover crop requires additional steps and the application of chemicals to the field.
It is an object of the present invention to provide a new means of reducing soil erosion and weed growth in crops, including potatoes. It is a further object of the invention to provide a means for reducing soil erosion and weed growth without the use of chemical herbicides.